Services used in mobile handheld terminals require relatively low bandwidth. Estimated maximum bitrate for streaming video using advanced compression like MPEG-4 is in order of a few hundred kilobits per second.
A DVB-T (Terrestrial Digital Video Broadcasting) transmission system usually provides data rates of 10 Mbps or more. This provides a possibility to significantly reduce the average DVB-T receiver power consumption by introducing a schema which can be based on time division multiplexing (TDM). The introduced scheme can be called a time slicing.
An idea of time-slicing is to send data in bursts using significantly high bandwidth at once. The enables a receiver to stay active only a fragment of the time, while receiving bursts of a requested service. An example of the time slicing can be depicted in FIG. 1. So the original possibly streaming data can be sent as burst with high bandwidth load. Two time slice bursts (100,101) are depicted each burst having their respective synchronization portion (102) and data portion carrying the service (103).
The received data can be buffered. For example, if an applicable constant lower bitrate is required by the mobile handheld terminal, this may be provided by buffering the received bursts. Thus the data used by the end-application can be applied even as a stream by unpacking data in the buffer(s).
For an exemplary burst size of 2 Mbit and a DVB-T bitrate of 15 Mbps, the burst duration is 146 ms. If the constant bitrate (the bitrate at which the burst is read out of the buffer) is 350 kbps (e.g. one streaming service with high quality video), the average time between bursts is 6.1 s.
As the total on-time is the addition of the synchronization time plus the burst duration, synchronization times of the handheld receiver must be rigorously minimized in order to better exploit the potential of time-slicing.
So the technical use of TDM based system such as time slicing to cut power consumption to a reasonable number is generalizing for a DVB handheld environment. Therefore, in order to better exploit the potential power reduction, synchronization times of such a receiver should be decreased. A faster synchronization is desirable.
An approach for a multi-carrier transmission synchronization according to the prior art, will hereinafter be described.
Typical DVB-T Synchronization According to Prior Art
A typical DVB-T synchronization scheme until Channel Estimation is sketched a in a standardization publication: “Digital Video Broadcasting (DYB)”, ETS 300 744, chapter 4.4 incorporated herein by reference. This typical synchronization scheme is depicted in FIG. 2. After start-up, the first step of synchronization is a Pre-FFT (Fast Fourier Transform) synchronization (200). As all metrics at this stage are derived from a guard interval correlation, a typical synchronization time of two OFDM (Orthogonal Frequency Division Multiplex) symbols is inherent.
For Subsequent Post-FFT synchronization (201), taking into account, that the first OFDM symbol is available for Post-FFT synchronization after the latency of the FFT (typically 3 OFDM symbols) a typical synchronization time of 4-5 OFDM symbols is related to this phase.
After carrier and timing synchronization have been achieved, the position of scattered pilots within an OFDM symbol has to be determined before the channel estimation can be started. As the scattered pilot position is directly related to the OFDM symbol number within the OFDM frame, no dedicated scattered pilot synchronization is typically included in prior art DVB-T receivers, but the anyhow available TPS-bit-based OFDM frame synchronization (202) instead. As a consequence, this implies a variable minimum synchronization time of 17 to 68 OFDM symbols. For DVB-H (DVB in handheld mobile terminal environment) time-slicing purposes this means the receiver must prepare for the later one, thus 68 OFDM symbols synchronization time have to be reserved. All in all, this accounts for 75 OFDM symbols synchronization time until Channel Estimation (CHE, 203) can be started. Assuming 8 k mode, this translates into 69-84 ms depending on the length of the guard interval. Taking only this part of the synchronization time (Channel Estimation omitted), already this 84 ms is quite impressive compared to 146 ms burst duration. 37% of the total on-time is just for this part of the synchronization, most of it resulting from the TPS (Transmission Parameter Signalling) synchronization (202).
Another approach for a multi-carrier transmission synchronization according to the prior art, will hereinafter be described.
A patent publication EP 1 267 535 A1 cites a prior art based approach relying on a system where scattered pilot detector decides the mode of each scattered pilot and compares the sums of powers of subcarriers corresponding to each scattered pilot mode with one another based on the feature that a pilot has greater power than usual data to detect a mode having greatest power among the modes.
In view of various limitation of the synchronization into a multi-carrier transmission, it would be desirable to avoid or mitigate these and other problems associated with prior art. Thus, there is a need for relatively fast and yet quite reliable synchronization.